This is junk science at its worst. In the first place, there is zero correlation between consumption of acidic substances and the pH at tissues. Hello, the pH of the stomach, with its HCl, is something like one, so there is no substance that one can consume that affects pH where digestion begins. A very strongly acidic environment is required for digestion, the composition of food substances is completely irrelevant to this process. Moreover, prganic acids like those present in green tea (and vinegar, e.g., which I never heard listed as a carcinogen), following absorption into the bloodstream, are easily buffered by substances in the blood, and when taken up by cells, by buffers in them, so they never affect tissue pH in the slightest.According to Dr. Robert O. Young, Director of Research at the pH Miracle Living Center, "green tea is acidic and will contribute to the dietary acid and metabolic loads in the tissue causing a cancerous condition or making a cancerous condition worse."
Even if they did, I'm not aware of any studies associating acidic pH or acided substances in tissues with carcinogenic effects. Carcinogenesis is promoted by certain substance that may result in gene mutations or other chromosomal abnormalities, or affect certain metabolic pathways in cells. These processes are extremely complex, and there is a great deal we still don't understand about them. The evidence to date is that certain substances in green tea are beneficial; it's possible that there are also substances in green tea that could be harmful. But their acidic nature is not the source of these effects. Much as Miracle Institutes would love you to believe otherwise, there is no simple solution like altering pH.